This invention relates to a printer, in particular to a single pass, multi-colour electrostatographic printer, and to a method of single-pass multi-colour electrostatographic printing.
Electrostatographic printers are known in which a toner image is electrostatically formed on a rotatable endless surface, such as a belt or a drum, and then ultimately transferred to a receiving material, which is usually in the form of paper sheets or a web.
U.S. Pat. No. 4,796,048 (Bean/Xerox Corporation) describes a copying apparatus in which a monochrome liquid toner image is formed on a photoconductor and then deposited on a transfer member in the form of a belt. The image is transferred from the belt to a substrate. In one described embodiment, the solvent in the liquid toner is removed from the toner image while it is carried on the belt by the application of infra-red radiation and a vacuum. The image is then transferred to the substrate by heat and pressure and the belt is then optionally cooled before a further image is deposited thereon.
International patent application WO92/10793 (Spectrum Sciences BV) describes an imaging apparatus in which a liquid toner image is formed on a single photoconductor and then deposited on a transfer member in the form of a heated transfer drum and transferred from there to a substrate. The surface of the heated transfer drum may be cooled in advance of the deposition of the image. The multiple image is deposited on the transfer drum in steps, that is the transfer drum is rotated once for each colour image being deposited. Cooling of the drum surface is necessary in advance of the deposition of each next colour image in order to avoid back transfer of the toner to the photoconductor. Step-by-step deposition is slow, in particular because of a speed limitation which is inherent in the image writing system. Where, for example, four colour images are deposited, the overall printing speed can be no faster than 25% of the image writing speed. Also, the apparatus described by Spectrum introduces the risk of contamination of one toner developing unit by toner of another colour. As a consequence, the apparatus described by Spectrum includes a very thorough cleaning system for the photoconductor.
In any event, we prefer to avoid the use of liquid toners as described in Bean and Spectrum referred to above, especially where such toners are based on non-aqueous solvents such as Isopar (Trade Mark), which is mainly decane. Such solvents may not freely be released into the atmosphere for environmental reasons and it is therefore necessary to include special arrangements to avoid such release.
Copiers and printers have been proposed which make use of toner in powder form. In U.S. Pat. No. 5,059,990 (Abreu et al./Xerox Corporation) for example, a multi-pass multi-colour printer is described in which a sheet of receiving material is moved in a recirculating path into contact with a single toner image carrying photoconductive belt, to which powder toner images of various colours are applied in turn. Such multiple-pass printers introduce considerable difficulties in the registration of the various toner images on the receiving material and also suffer from similar speed limitations to those referred to above in connection with the apparatus described by Spectrum.
U.S. Pat. No. 5,119,140(Burkes et al./Xerox Corporation) describes a printer in which a number of powder toner images are deposited in turn onto an image receiving member to form a multiple toner image thereon. The multiple toner image is thereafter transferred by electrostatic means to a plain paper substrate. The efficiency of the electrostatic transfer to the substrate is dependant upon the nature and condition of the substrate and may not be 100% effective. For this reason Berkes et al. require the provision of a device for cleaning the image receiving member before a further image is deposited thereon.
In European patent application EP 220663A (Colorocs Corporation), a single pass, multi-colour printer is described in which a multiple toner image is formed on a transfer belt and then transferred to a substrate, normally in the form of a sheet of paper. The multiple toner image is formed on the transfer belt by sequential transfer from a photoreceptor belt onto which toner images of different colours are formed by electrostatographic means. In order to form the multiple toner image, the transfer belt has to circulate a number of times, corresponding at least to the number of different colour toner images, before the multiple toner image can be transferred to the paper sheet. This construction introduces considerable problems in ensuring accurate registration of the different coloured images and speed limitations as discussed above in connection with the apparatus described by Abreu et al.
In U.S. Pat. No. 5,455,668 (De Bock et al./Xeikon NV) a single-pass multi-colour printer is described in which a substrate in the form of a web passes a plurality of toner image forming stations where images of different colours are simultaneously transferred thereto in register.
Once one or more toner images have been transferred to the substrate, it is necessary to fix the images thereon. A number of fixing techniques are known, such as radiant heat fixing, and hot or cold pressure fixing. Radiant fixing has advantages of not introducing contact with the substrate but consumes significant energy, its efficiency is dependant upon the nature and characteristics of the substrate, questions may arise concerning the evaporation of environmentally unacceptable compounds which may be present in the substrate and the dry substrate may suffer from dimensional instability resulting in wrinkling and can become easily charged resulting, for example, in stacking problems. Where the thermal expansion coefficients of the substrate and the toner are significantly different, the use of radiant fixing can lead to distortion of the final printed image. Furthermore, radiant fixing is less suitable for substrates in the form of cut sheets as opposed to a web, since the position of the substrate path is more difficult to ensure. Pressure roller fixing on the other hand, while consuming less energy, is a contact method and the rollers used have a relatively short life-time. Furthermore, pressure roller fixing often requires the use of liquid release agents, such as silicone oils, while it is preferred to reduce the level and variety of consumable materials used in the printer.